Heretofore, radio receivers having trigger circuits associated therewith have employed voltage divider type circuits connected to a detector output to divert a portion of the complete detected signal to the trigger circuit. Consequently, a reduced portion of the complete detected signal is applied to an audio output stage, and thus the audio output circuit must provide greater amplification to achieve the same level audio output. These circuits require additional amplifier and filter stages, adding to radio receiver cost and impose an increased load on battery operated receivers.
An example of one such divider circuit is shown in U.S. Pat. No. 3,896,386, Tuning Indicator With Noise Signal Indicator. Referring to FIG. 2 thereof, which is a diagram of an FM receiver incorporating an indicator circuit, the detector 16 is connected in parallel to both the stereo demodulator and to the input 23 of the noise selector circuit 22. Consequently, the signal supplied to the stereo demodulator 17 is the detected signal less that portion of the detected signal which is applied to the noise selector circuit 22 through input 23. U.S. Pat. No. 3,896,386 does not disclose the value of the capacitor connected between the FM detector and the preamplifier. A large value capacitor unnecessarily loads the detector output while a small value capacitor does not extract a signal of useable magnitude. A preamplifier will thus be required when a small valued capacitor is used. Further, the noise selector circuit 22 does not utilize all the extracted signal, but instead amplifies only that portion of the signal within the pass band of a tuned circuit 24. The noise selector circuit 22 of the U.S. Pat. No. 3,896,386 includes components such as additional resistors, capacitors and transistors not required in the selector circuit of the present invention, and the selector circuit 22 imposes a load on the power supply which is greater than the load imposed by the selector circuit of the present invention.